Police said several businesses have accepted some of the counterfeit bills. Investigators said there have been almost a dozen cases of the counterfeit bills being passed since late December.

The most recent incident happened Wednesday at Bunny's Superette on Webster Street when a woman tried to give the clerk a phony bill.

"A woman had come into the store and presented her with a $100 bill to make a small purchase," said Lt. Maureen Tessier. "The clerk pretty much right off the bat recognized that the bill was counterfeit and ultimately confronted the customer about that. The woman reportedly acknowledged that it was counterfeit and left the store."

Police said the woman left the bill behind.

In another case, Brandon Ross, 22, was charged on Thursday with two counts of forgery and two counts of theft by deception. Police said he used counterfeit cash to make purchases at A&A Convenience on Hall Street on Feb. 8.

Amer Jacob of A&A Convenience said the $50 and $100 bills that Ross is accused of using at his store passed the marker test for authenticity, but he said a closer look showed the bill had been altered.

"The pen shows me it's real, but if you look really close and deep, you can tell it's a $1 bill that was washed out," he said.

Police said the use of larger counterfeit bills is new. Most of the incidents at places such as McDonald's and Dunkin' Donuts have been $20 bills.

"We're just encouraging clerks and store owners to closely examine currency before they conduct a transaction," Tessier said. "If they have any doubts, they certainly shouldn't accept the money as payment."

Police said they believe at least two of the counterfeit transactions are related and are working to determine the source of the fake money.